Natural gas is a fast growing segment in the alternative fuel market for transportation in the United States and in other countries that have a surplus of natural gas. It is an alternative fuel that replaces gasoline, diesel and/or blends with diesel to power automobiles, pickups, light to heavy trucks, busses and high horsepower applications such as oil field drilling operations. Compressed natural gas (CNG) is dispensed to vehicles or other portable tanks after it has been stored in pressure vessels or it can be directly filled from the CNG compressors.
Compressed natural gas is not dispensed by pumping gas from a liquid reservoir like traditional gasoline and diesel pumps. Instead, CNG fueling stations dispense fuel under very high pressures and use pressure differentials to move the gas; gas flows from an area of higher to lower pressure.
The most typical configuration of a “fast fill” CNG station uses one or more compressor(s) to fill vehicles or other portable tanks and supplements the filling operation with a cascading storage through a priority panel. The priority panel is a network of tubing and valves plumbed together that receive gas from the compressor(s) and distributes CNG to storage, from storage to dispenser, or directly from compressor to dispenser according to the controls applied. A cascading storage system will typically have three banks of pressure (high, middle, and low). When the pressure falls below certain presets within the storage banks, the priority panel distributes CNG to fill each bank of the cascading storage system. When the pressure falls too low in the high bank storage, the priority panel will also close off the storage banks and distribute CNG directly from the compressor to the fuel dispensers through the high bank fill manifold.
The cascading storage system allows each dispenser to pull CNG first from the low bank, then switch to the middle bank, and lastly switch to the high bank to maintain the highest differential of pressure allowing for complete fills without starting a compressor for every vehicle filling. A site controller can be utilized to manage the function of the priority panel as well as control each compressor in a system. Each dispenser can take fuel from each bank of storage simultaneously, and are all plumbed in the high, middle, and low CNG distribution manifolds between the priority panel and the dispensers.
The issue with this type of system is that all of the dispensers in a system share a common bank of storage and direct fill. So when more than one vehicle is filling up on the system at the same time, the vehicle or other portable gas tank with the lowest gas pressure gets CNG the fastest while the vehicle or other portable gas tank closest to a complete fill (and hence with higher pressure) will slow or stall until the lower pressure vehicle or tank fills with enough CNG to equalize the pressure.
An example illustrating the multiple vehicle filling problem is when a first vehicle—say a large truck—pulls in and begins fueling, pulling a large volume of CNG from the cascading system before a compressor comes online to fill the storage or truck directly. As the first vehicle is still filling, another vehicle (or other gas tank is brought in) pulls in and begins fueling. When the two vehicles begin flowing gas from the high bank storage, the dispenser connected to the vehicle with greater pressure in the fuel tank will stall until the pressure in the tanks equalize. After the fuel tanks equalize they will be sharing CNG flow and the fill volume is split in half. If yet another vehicle begins fueling at the same time, the fill volume is effectively cut in third, which results in long fill times during peak demand periods.
Lacking in prior art is a way for distribution of CNG in fueling stations to maintain high differential of pressure through prioritized individual dispensers with multiple fueling vehicles.